IDENTIFICATION OF 2-FORMS OF GLUTAMINE-SYNTHETASE IN BARLEY (HORDEUM-VULGARE)

Two forms (designated GSI and GSII) of glutamine synthetase (E.C.6.3.1.2) have been found in barley (Hordeum vulgare) which can be separated by ion exchange chromatography. Light grown shoots contain both GSI and GSII, roots, seeds and etiolated shoots exhibit only GSI activity. In light grown shoots GSII appears to be chloroplastic and GSI cytosolic. Sedimentation coefficients of both forms are similar (GSI, 14.04; GSII, 14.13 S20w) as are their Stokes radii (GSI, 6.13; GSII, 6.35 nm) and molecular weights (GSI 349,000; GSII 363,000 daltons). The pH optimum for GSII (pH 7.5) is somewhat more alkaline than that of GSI (pH 7.0). GSII is less stable than GSI; in the absence of protective ligands it loses activity at 30°C and is inactivated by thiol reactive agents. GSI is stable at 30°C and its activity is unaffected by thiol reactive agents. © 1979.